<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>三列</title>
  <style>
    #content:after{
      content: '';
      display: block;
      clear: both;
    }
    .menu{
      width: 100px;
      height: 500px;
      background: green;
      float: left;
    }
    .aside{
      width: 200px;
      height: 500px;
      background: yellow;
      float: right;
    }
    .main{
      margin-left: 110px; 
      margin-right: 210px;
      height: 400px;
      background: red;
    }
    
    #footer{
      background: #ccc;
    }
  
  </style>
</head>
<body>
  <div id="content">
    <!-- 为什么不是main在前面 -->
    <div class="menu">aside</div>
   
    <div class="aside">aside</div>
     <div class="main">content</div>  
  </div>
</body>
</html>
